Rotational isomerism of acetic acid isolated in rare-gas matrices: Effect of medium and isotopic substitution on IR-induced isomerization quantum yield and cis→trans tunneling rate

Abstract

Redistribution of carbonyl stretch mode energy in isolated and solvated N-methylacetamide: Kinetic energy spectral density analyses J. Chem. Phys. 135, 214504 (2011) A general and efficient Monte Carlo method for sampling intramolecular degrees of freedom of branched and cyclic molecules J. Chem. Phys. 135, 134121 (2011) Instanton calculations of tunneling splittings for water dimer and trimer J. Chem. Phys. 135, 124109 (2011) Intramolecular vibrational dynamics in S1 p-fluorotoluene. I. Direct observation of doorway states J. Chem. Phys. 135, 124305 (2011) On the stability of Be3: A benchmark complete active space self-consistent field + averaged quadratic coupled cluster study J. Chem. Phys. 135, 104311 (2011) Additional information on J. Chem. Phys. Rotational isomerization of acetic acid (CH 3 COOH) is studied in Ar, Kr, and Xe matrices. The light-induced trans→cis reaction is promoted using resonant excitation of a number of modes in the 3500-7000 cm Ϫ1 region, and the quantum yields for this process are measured for various acetic acid isotopologues and matrix materials. For excitation of acetic acid at energies above the predicted isomerization energy barrier ͑у4400 cm Ϫ1 ͒, the measured quantum yields are in average 2%-3%, and this is one order of magnitude smaller than the corresponding values known for formic acid ͑HCOOH͒. This difference is interpreted in terms of the presence of the methyl group in acetic acid, which enhances energy relaxation channels competing with the rotational isomerization. This picture is supported by the observed large effect of deuteration of the methyl group on the photoisomerization quantum yield. The trans→cis reaction quantum yields are found to be similar for Ar, Kr, and Xe matrices, suggesting similar energy relaxation processes for this molecule in the various matrices. The IR-induced cis→trans process, studied for acetic acid deuterated in the hydroxyl group, shows reliably larger quantum yields as compared with the trans→cis process. For pumping of acetic acid at energies below the predicted isomerization barrier, the trans→cis reaction quantum yields decrease strongly when the photon energy decreases, and tunneling is the most probable mechanism for this process. For the cis→trans dark reaction, the observed temperature and medium effects indicate the participation of the lattice phonons in the tunneling-induced process.

Trans-cismolecular photoswitching in interstellar space

“…First, the isomerization of solid HCOOH after IR irradiation of icy grain surfaces (as observed in the laboratory, Maçôas et al 2004;Olbert-Majkut et al 2008) and subsequent desorption to the gas-phase. Second, the gas-phase isomerization by collisions of HCOOH with energetic electrons (∼0.5 eV).…”

“…In the laboratory, trans-to-cis isomerization has been observed in molecular ices irradiated by near-IR photons (Maçôas et al 2004;Olbert-Majkut et al 2008). Hence, isomerization of solid HCOOH and subsequent desorption to the gas-phase might also be responsible of the cis-HCOOH enhancement.…”

“…Hence, isomerization of solid HCOOH and subsequent desorption to the gas-phase might also be responsible of the cis-HCOOH enhancement. However, owing to the short lifetime of the cis conformer observed in ices (a few minutes if the irradiation is stopped, Maçôas et al 2004), a very strong flux of IR photons would be needed to maintain significant abundances of solid cis-HCOOH. In addition, near-IR photons penetrate molecular clouds much deeper than FUV photons, and one would have expected to detect cis-HCOOH in all positions of the Bar, and towards the Orion hot core, a region irradiated by intense IR fields.…”

“…This barrier can be overcome by light. Photoisomerization (or photoswitching) has been studied in ice IR-irradiation experiments (e.g., Maçôas et al 2004), in biological processes, and, for large polyatomic molecules, in gas-phase experiments (Ryan & Levy 2001). HCOOH is the simplest organic acid and has two conformers (trans and cis) depending on the orientation of the hydrogen single bond.…”

Formic and acetic acids in a nitrogen matrix: Enhanced stability of the higher-energy conformer

“…Hydrogen tunneling in FA and AA has been the subject of several studies. 9,[15][16][17][18][19] cis-CH 3 COOH has a shorter lifetime in rare-gas matrices than cis-HCOOH ͓approximately 50 s ͑Ref. 18͒ versus 8 min ͑Ref.…”